We propose to study the mechanism of DNA replication. Our ultimate purpose is to understand the control of chromosome duplication. In order to do this we wish to characterize replicating DNA molecules and to understand in detail whether or not the rolling circle model provides an adequate description of these structures. We propose to examine several systems: the replication of the virus phi X 174, of phage lambda, phage T7, and the replication of mitochondrial DNA. Our basic approach is to follow radioactive tracers incorporated in vivo, to isolate DNA molecules in intermediate stages of replication, and to characterize these molecules by physical-chemical and electron- microscopic techniques.